The present invention relates to a frequency offset detection processing system and a frequency offset detection processing method using the same.
CDMA (Code Division Multiple Access) is one of communication schemes currently used in mobile communication systems. CDMA is a communication system in which data is spread on the transmitting side by using a code, and the transmitted data is decoded by despreading it on the receiving side by using the same code. In a mobile device, AFC (Auto Frequency Control) control is performed to make the reference frequency of the base station coincide with that of the mobile device. In AFC, the shift (frequency offset) between the reference frequency of the base station and that of the mobile device is detected from a pilot signal transmitted from the base station, and correction is executed. When a frequency offset exists, phase movement occurs between the symbol of an immediately preceding pilot signal and the current pilot signal (FIG. 7). On the basis of a phase moving amount θ, the frequency offset can be calculated byΔf=Rate×(θ/360°)(Δf: frequency offset, θ: phase moving amount between symbols of pilot signals, Rate: symbol rate of pilot signal)
In actual radio communication, the phase of a reception signal varies due to white noise or fading. Accordingly, the accuracy of the phase moving amount detection value by a frequency offset decreases. To solve this problem, there is a method of averaging phase moving amount detection values by a plurality of frequency offsets to increase the detection accuracy. FIG. 8 shows the averaging processing. Let θ1, θ2, . . . θn be phase moving amount detection errors by a frequency offset. A detection error after the averaging processing is given by
      θ    ⁢                  ⁢    α    =            ∑              k        -        1            n        ⁢                  θ        k            /      n      
Since the values θ1 to θn are random, they are time-averaged to 0. A detection error θα after the averaging processing is smaller than that before averaging at a high probability. For this reason, the detection error can be reduced. The theoretical frequency offset detection limit value corresponds to the phase moving amount θ within the range of −180° to +180°. If θ exceeds ±180°, the direction of shift of the frequency offset is erroneously determined. At this time, the sign of the phase moving amount detection value by the frequency offset is different from the actual sign of the phase moving amount detection value by the frequency offset. For example, when the actual phase moving amount is −190°, the detection value is +170°, i.e., the sign is erroneously determined. Under the influence of noise or fading, even when the phase moving amount does not exceed the range of −180° to +180°, the sign may erroneously be detected.
FIG. 9 shows this situation. The phase moving amount by a frequency offset is +170°. When the phase rotates by +25° due to noise, the phase moving amount is +195°. However, the detection value is −165°, i.e., an error occurs. If a detection value with a false sign is included in detection values to be added in averaging processing, the detection accuracy greatly degrades. Referring to FIG. 10, when some detection values (1), (2), and (3) have a false sign, an average θa of the detection values greatly changes from an actual phase moving amount θf.
To solve the above-described problem and increase the accuracy for detecting the shift direction of a frequency offset in AFC control, a method of executing majority determination for the detection values of the shift direction of the frequency offset is used (Japanese Patent Laid-Open No. 9-233139). In this method, to separately process the magnitude and direction of phase shift data, an automatic frequency control apparatus having a means for extracting only the magnitude information of a phase shift, a means for averaging only the magnitude information of the phase shift, a means for extracting only the direction information of the phase shift, and a means for making decision by majority for the direction information of the phase shift and selecting a direction of majority is used. The frequency of a local generation unit is controlled in accordance with the average value and majority.
Thermal noise added to a reception signal in radio communication normally has a Gaussian distribution. When measurement is executed for a sufficiently long time, the central value of the distribution of the phase shift detection values and the actual phase shift have an almost same value (FIG. 11A). However, in the above-described method described in Japanese Patent Laid-Open No. 9-233139, the distribution of phase shift detection values after conversion by the majority determination result is distorted. Hence, the central value (≈actual phase shift) of the distribution and the average value (=detection value after processing) of the distribution have different values (FIG. 11B). Hence, the phase shift detection error due to interference of thermal noise or the like cannot sufficiently be reduced.